The invention pertains to a composition and a process for plugging a subterranean void where the composition will be subjected to an in situ temperature of about 150.degree. C. or greater in said void. The composition upon hydration at or above the designated temperature exhibits expansive properties making it especially useful for filling the annulus between the casing and the wellbore of a geothermal well. Upon curing, the expansion of said composition provides a tight seal between the casing and the formation and thus prevents communication of fluids between the different zones of the subterranean formation that the wellbore traverses.
Cement compositions capable of expansive behavior when slurries thereof are hydrated have been prepared and used for plugging of subterranean voids, e.g. the annulus between the casing and wellbores of gas, oil and water wells. Such compositions have heretofore depended upon the interaction of the calcium and silicon components of hydraulic cements with sulfate-containing compounds such as gypsum and plaster of Paris. Chem Comp cement is a commercial cement of such a nature. However, when slurries of such expanding cements are subjected to temperatures in excess of about 100.degree. C. they lose their expansive capability.
Nelson and Kalousek, in Cement and Concrete Research, Volume 7, page 687 (1977) have discussed the effect of sodium oxide and sodium salts on calcium silicate hydrates at temperatures in the range of 108.degree. to 325.degree. C. They discuss the formation of pectolite from various ratios of calcium/silicon mixes to which the sodium ion is added in the form sodium hydroxide, sodium carbonate or sodium sulfate. Expansive properties of such pectolite-containing materials are not mentioned.